The present invention relates to a method of processing uraniferous ores, particularly refractory ores, whose uranium cannot be practically dissolved by conventional acid processing, either because the consumption of reagent is excessive, or because the reaction is so slow that it is not economically feasible, or because it is not possible to achieve solubilization of a sufficient uranium proportion.
In such ores, the greater part of uranium is generally present at the valence 4 as fine mineralization and is often associated with chemically reducing compounds such as organic substances and sulphides. In such ores, carbonates are also frequently found, and have an unfavourable action on the conventional methods.
A prior art method of processing such ores comprises comminuting the ore, mixing it with a concentrated sulphuric acid solution, the initial volume of H.sub.2 SO.sub.4 solution added per unit weight of ore being chosen in order to obtain a product which behaves as a solid phase and the sulphuric acid content being such that a residual acidity, after forming into paste, is obtained, comprised between 10 g/l and 40 g/l. The impregnation operation is followed by curing of the product for several hours, while the product is maintained at the consistency of a wet solid (that is with a low liquid volume/solid weight ratio) and during which the attack of the uraniferous chemical compounds, started during impregnation, is completed. The uranium, present at the valence 6 in the cured product, is diluted to the pasty state, which is then subjected to a conventional liquid-solid separation.
The three phases of impregnation or wetting, curing and forming into paste defined above are generally carried out under the following conditions.
Impregnation: The comminuted ore obtained by dry grinding is impregnated with a small quantity of strongly concentrated aqueous sulphuric acid solution, with a liquid volume per unit weight of ore such that the resulting product does not behave like a paste, but like a scarcely moist solid, in such a way that the product sticks as little as possible to the walls of the apparatus in which it is processed. In practice, the ground mineral is typically impregnated with from 60 to 150 l/ton of ore of a solution of 50% water and 50% sulphuric acid. The operation is generally carried out continuously in a rotating drum mixer; its duration is normally from 10 to 15 minutes and should not exceed 30 minutes, for removing the need for drums too long for operation of a scaper within the drum. The impregnation is carried out with spraying nozzles distributing the solution in the drum. In the place of a drum, other types of equipment can be used.
Ripening or curing: Curing of the product which has a low solution content is carried out under static or dynamic condition for several hours, until the attack of the uraniferous constituents, begun during impregnation, has been completed. Curing can be static on a curing conveyor belt of large width, whose length can be more than 100 meters. The ore is then poured as a thick layer from the outlet of the impregnator. Curing can as well be carried out dynamically in a rotating drum of large size.
Recovery and pulping: The uranium--including that brought from valence 4 to valence 6 in the cured ore--is in solution in the interstitial liquid or in the form of crystallized and easily soluble salts. During recovery, the product is changed into a paste so that the uranium is entirely retained in a solution which can be separated from the solids by a conventional method such as settling or filtration.
The recovery can include a disaggregation of the granules formed during impregnation and curing, for example in a pebble crusher fed with solution. It can subsequently comprise a final step of elution in an agitated tank. A time period not exceeding some tens of minutes at a moderate temperature (for example 50.degree. C.) is sufficient to recover the uranium as a solution.
While the steps of impregnation and of forming into a pulp or paste do not raise technological difficulties this is not so as regards curing. Curing requires very heavy investments and the material (typically elastomer) of the conveyor belts used for static curing is subject to rapid wear due to the high temperature. In this respect, it should be noted that impregnation is carried out at a relatively high temperature due to several factors which cause heating:
dry-grinding or crushing necessitates heating to dry the ore and possibly improve separation according to grain size at the output,
mixing sulphuric acid with the quantity of water necessary to effect good impregnation results in the adjunction of the heat due to dilution of sulphuric acid, if the two liquids are mixed inside the impregnator,
reactions of H.sub.2 SO.sub.4 with the uranium containing compounds and with the other acid-consuming components (such as ferromagnesium silico-aluminates which are present in practically all natural ores and carbonates), are exothermic,
when addition of an oxidizer is necessary for uranium to be rendered soluble, oxidation of the reducing agents contained in the ore, such as sulphides and organic substances, is also exothermic.
In practice, the curing temperatures are never lower than 80.degree. C. and they can be much higher if the ore contains considerable quantities of compounds reacting with the sulphuric acid and the oxidizing agent.
If a rotary drum is used for curing rather than a belt, the drum is so long that it cannot be provided with scrapers and it is necessary to limit the liquid/solid ratio to values not exceeding 0.1. But for ores consuming a great deal of acid (typically more than 60 kg/t), the per weight ratio of water/sulphuric acid is thus too low to achieve optimum efficiency.
In addition, experience has shown that in numerous cases the usable oxidizing agents are very few. Referring to those ores in which uranium is present in the silico-aluminate base or matrix which cements quartz particles of a sandstone and whose deposits are located in regions where the cost of transportation is high, manganese dioxide is very expensive, has a low efficiency and consumes sulphuric acid; most of sodium chlorate decomposes into chlorine and chlorine dioxide without oxidizing the ores at the start of impregnation, that is to say when the acid is very concentrated. Only nitric acid and nitrates are economically of interest. Nitrous vapors resulting from oxido-reduction reactions can be reoxidized (by air in most cases) and reconverted into nitric acid or nitrates, possibly containing nitrites, which can then be recycled to the impregnation stage. In the case of ores containing considerable quantities of reducing compounds such as, for example, most of the ores found on the territory of the Republic of Niger, the nitrates are entirely and rapidly decomposed during the impregnation step; consequently, the liquors which have been subjected to solid-liquid separation and are circulated to a stage where ion-exchange is carried out, contain only negligible amounts of nitrate.